Journal of Materials Science & Technology, 2020, 54(0): 171-180 DOI: 10.1016/j.jmst.2020.02.005

Research Article

Effect of Ni content in Cu1-xNix coating on microstructure evolution and mechanical properties of W/Mo joint via low-temperature diffusion bonding

Mei Raoa, Guoqiang Luo,a,*, Jian Zhanga, Yiyu Wangb, Qiang Shena, Lianmeng Zhanga

a State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, 430070, China

b Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831-6064, USA

Corresponding authors: *E-mail address:luoguoqiang1980@sina.com(G. Luo).

Received: 2019-07-18   Accepted: 2019-11-21   Online: 2020-10-1

Abstract

The 93W and Mo1 refractory metals were bonded with different Cu1-xNix coating interlayers of various Ni content using plasma-activated sintering at 700 °C. The effects of the Ni content in the Cu1-xNix coating interlayer on the interfacial microstructure evolution and mechanical properties of the W/Mo joints were studied. The maximum average shear strength of the W/Mo joint was 316.5 MPa when the Ni content of the Cu1-xNix coating interlayer was 25 %. When the Ni content of the Cu1-xNix coating interlayer was below 50 %, the atomic diffusion at the W/Mo joint interface was adequate without the formation of intermetallic compounds, as demonstrated by the High Resolution Transmission Electron Microscope analyses of the joints. The presence of Ni in Cu1-xNix promoted diffusion bonding at the interface, which contributed to the high mechanical properties of the W/Mo joint. With an increase in the Ni content of the Cu1-xNix coating interlayer, the MoNi intermetallic compound (IMC) nucleated and grew at the Cu1-xNix coating/Mo1 interface. When the Ni content of the Cu1-xNix coating interlayer was above 50 %, the generation of a brittle MoNi IMC weakened the shear strength of the W/Mo joint dramatically.

Keywords: Refractory metal ; Cu1-xNix coating interlayer Solid solution ; MoNi IMC ; Microstructure ; Shear strength

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Cite this article

Mei Rao, Guoqiang Luo, Jian Zhang, Yiyu Wang, Qiang Shen, Lianmeng Zhang. Effect of Ni content in Cu1-xNix coating on microstructure evolution and mechanical properties of W/Mo joint via low-temperature diffusion bonding. Journal of Materials Science & Technology[J], 2020, 54(0): 171-180 DOI:10.1016/j.jmst.2020.02.005

1. Introduction

The W and Mo alloy refractory metals exhibit a desirable combination of properties, including high melting points, high densities, good mechanical properties at room and high temperatures, low thermal expansion values, and good resistances to corrosion [[1], [2], [3]]. As a result, W and Mo alloys show great potential for aerospace, automotive, metal working, military, plasma-facing component, and nuclear technology applications [[4], [5], [6]]. Reported investigations on the joining of W and Mo alloys have shown their great potential value for impregnated dispenser cathodes and other future applications. However, W and Mo alloys have differences in their melting points, thermal expansion, and thermal conductivity. Thus, the fabrication of a sound W/Mo joint is challenging, and efforts have been made to address this issue [7,8].

Diffusion bonding is a conventional approach to bonding dissimilar materials, especially in joining refractory metals, because of its lower bonding temperature, good repeatability, and small thermal stress on base metals [9]. Recently, some studies have demonstrated the diffusion bonding of molybdenum alloys and tungsten alloys [[10], [11], [12], [13], [14]]. Usually, pure metal foils, mixture powders, alloy foils, and thin films are selected as the bonding interlayers to obtain high-quality joints at low temperatures. Lin revealed that porous W and Mo could be bonded using Ni foil and Pd foil as the interlayer, with the results showing that a remarkable joint, free of any defects, could be successfully prepared using a temperature of 1610 °C for 10 min by adding Pd foil [10]. In another study [11], a Ni-Mo mixture powder bonding interlayer was applied to join porous W and Mo alloys at 1400 °C for 5 min. The average tensile strength of the joint was 87.9 MPa. The reliable bonding of the TZM alloy and ZrCp-W composite was achieved by adding Ti-35Ni and Ti-61Ni eutectic alloys at 1240-1260 °C, achieving optimal shear strengths of 123.8 MPa and 124.8 MPa at room temperature, respectively [12,13]. In our previous studies, the 93W and Mo1 alloys were successfully bonded using a pure Cu film interlayer at low temperature, and the maximum bonding strength was 212 MPa [14].

Adding a coating or film interlayer is very important when bonding dissimilar materials. Coating interlayers can enhance the surface-to-surface contact with the base materials, reduce the residual stress, and prevent the generation of brittle intermetallic compounds. Therefore, the deposition of a metal or alloy coating as a bonding interlayer is an effective approach to minimize the bonding temperature and enhance the bonding strength of joints. By employing a Ni film and Ti foil multilayer, SS304L and Zircaloy-4 were bonded at 850 °C for 60 min, and the bonding strength of the joints was 209 MPa [15]. An Al/Ni film interlayer significantly decreased the bonding temperature and improved the bonding strength of ceramic Al2O3/Cu joints [16]. A silver film interlayer prevented the appearance of brittle Mg-Al compounds and increased the bonding strength of Mg/Al [17].

Recently, plasma-activated sintering (PAS) has been applied as a rapid joining process to bond dissimilar metals. The dissimilar metals Cu/Al [18], Mg/Al [19], and Fe/Al [20] have successfully been bonded using PAS. The W/Ti multilayer composites were joined by PAS, which lowered the bonding temperature and prevented the grain growth in the W plates. As a result, W/Ti multilayers with better properties were fabricated [21].

In our previous studies, Cu films were used as the interlayer to bond the 93W and Mo1 alloys using the PAS apparatus. It was found that the Fe-Ni binder in the 93W base metal could diffuse into the Cu film and form solid copper solutions, which promoted atomic diffusion at the Cu film/Mo and W/Cu film interfaces, contributing to the remarkable shear strength of the joints at low bonding temperatures [14]. According to the Cu-Ni binary alloy phase diagram [22], nickel can dissolve in copper and form a solid solution in any proportion. The stacking fault energy of the copper-nickel alloy is larger than that of commercial pure copper; consequently, the copper-nickel alloy possesses superior mechanical properties [23]. In view of the above, we prepared Cu1-xNix (x = 0, 0.25, 0.5, 0.75) coating interlayers to study the effects of the Ni content in the Cu1-xNix coating interlayer on the interfacial microstructure evolution and mechanical properties of dissimilar W/Mo joints formed via low-temperature diffusion bonding. In addition, the fracture surfaces of the joints were also observed and are discussed.

2. Experimental procedure

The base materials employed in this study were Mo1 and 93W alloys supplied by Beijing Topone Tungsten & Molybdenum Technology Co. Ltd. The microstructures of the Mo1 and 93W alloys are presented in Fig. 1, and the chemical compositions of the base metals are listed in Table 1. The 93W and Mo1 bulk metals were cut into wafers with a diameter of 25 mm and a thickness of 8 mm via a wire electrode cutting machine. Then, the sliced 93W and Mo1 wafers were ground with SiC grit papers in the proper order, polished with a diamond-polishing solution (0.05 μm), and finally cleaned in absolute alcohol by ultrasonic cleaning.

Fig. 1.

Fig. 1.   Diagram of sample for diffusion bonding magnetron sputtering.


Table 1   Chemical compositions of 93W and Mo1.

MaterialsElemental Composition (at.%)
WMoFeNiTiV
93WBal.<0.0234<0.01<0.01
Mo1<0.02Bal.<0.02<0.01-<0.01

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In our previous studies, we investigated the influences of the bonding parameters on the microstructure and mechanical properties of 93W/Cu coating/Mo1 joints. When bonded at 700 °C for 20 min at 20 MPa, the joints reached their maximum bonding strength. Therefore, the bonding temperature of 700 °C, holding time of 20 min, and bonding pressure of 20 MPa were selected as the bonding parameters in this study. A total of four Cu1-xNix coatings were sputtered with four compositions: Cu-0 at.% Ni, Cu-25 at.% Ni, Cu-50 at.% Ni, and Cu-75 at.% Ni. These coatings are referred to as Cu, Cu0.75Ni0.25, Cu0.5Ni0.5, and Cu0.25Ni0.75, respectively. The Cu1-xNix coatings were plated on the burnished surfaces of the base materials (shown in Fig. 1) using a magnetron sputtering system engineered by the Kurt J. Lesker Company. During the magnetron sputtering process, the argon atmosphere was under a constant pressure of 1 Pa, and the base metals were heated to 500 °C for 20 min. The power was 100 W, and the deposition time was 30 min.

The deposited base metals were piled into a sandwich and packed into a graphite mold and punches. A PAS machine (model: Ed-Pas (Japan)) was used during the diffusion bonding procedure at a heating rate of 100 °C/min to 680 °C, then at a rate of 1-10 °C/min up to 700 °C for 20 min in a vacuum of 10 Pa under a bonding pressure of 20 MPa, and finally, a free cooling stage.

The interfacial structure of the joints was observed using field emission scanning electron microscopy (FESEM, FEI Quanta 250FEG) and transmission electron microscopy (TEM, FEI Talos F200). The elemental distribution of the bonded zone was analyzed with a JXA-8100 electron probe microanalysis instrument (EPMA) and a TEM apparatus fitted with an energy dispersive X-ray analyzer. The shear strength of each sample was tested using an MTS-810 universal testing machine at a loading rate of 0.5 mm/s at room temperature. The fracture surface morphologies and elemental distributions were characterized using the FESEM equipped with energy-dispersive spectrometry (EDS) capabilities. The phase compositions of the fracture surfaces were verified via an X-ray diffraction (XRD) system (model: Rigaku Ultima III (Japan)).

3. Results and discussion

3.1. Characterization of base metals and Cu1-xNix coatings

The surface morphologies of the four Cu1-xNix coatings on the Mo1 base metal are displayed in Fig. 2. It can be seen that the surfaces of the coatings are grains that generally have long columns. The thicknesses of the Cu1-xNix coatings are similar at nearly 2 μm (shown in Fig. 3), and the content of the Cu, Ni, and Mo elements have gradient distributions at the interface, indicating that atomic diffusion occurred between the Cu1-xNix coatings and base metal during the deposition process. The XRD patterns indicate that the Cu1-xNix coatings are highly crystallized with fcc crystal structures (Fig. 4). Compared with the Cu peaks, the Cu0.75Ni0.25, Cu0.5Ni0.5, and Cu0.25Ni0.75 peaks have slight deviations due to the lattice distortion caused by the solid solution of Ni atoms in the copper.

Fig. 2.

Fig. 2.   Surface morphology of Cu1-xNix coatings deposited on Mo1 alloy: (a) Cu, (b) Cu0.75Ni0.25, (c) Cu0.5Ni0.5 and (d) Cu0.25Ni0.75.


Fig. 3.

Fig. 3.   Interfacial morphology and elemental distributions of Cu0.75Ni0.25 coating on Mo1 alloy: (a) interfacial morphology, (b) line scanning and (c-e) area scanning of the Cu0.75Ni0.25 coating/Mo1 interface.


Fig. 4.

Fig. 4.   XRD patterns of Cu1-xNix coatings despoited on Mo1 alloy.


3.2. Microstructure of W/Mo joints with Cu1-xNix coating interlayers

The microstructures of the W/Mo joints with the Cu1-xNix coating interlayers are illustrated in Fig. 5. When the Ni content in the Cu1-xNix coating was below 50 %, W/Mo alloy joints with no micro-voids or cracks were fabricated (Fig. 5(a-c)), suggesting that enhanced physical joining and fabulous metallurgical bonding were achieved. When the Ni content of the Cu1-xNix coating was 75 %, there were cracks at the interface of the W/Mo alloy joint. Fig. 6 shows the line scan-EDS results for the W/Mo joints with the Cu0.5Ni0.5 and Cu0.25Ni0.75 coating interlayers, indicating variations in the concentrations of the tungsten, molybdenum, copper, and nickel base metals and interlayers. As shown in Fig. 6(a-c), five areas (marked I, II, III, IV, and V) are observed at the W/Mo boundary with the Cu, Cu0.25Ni0.75, and Cu0.5Ni0.5 interlayers. Regions I, III, and V are the 93W, a CuNi layer, and the Mo1 base metal, respectively. At the 93W-Cu1-xNix and Cu1-xNix-Mo1 (x ≤ 0.5) boundaries, the W, Cu, Ni, and Mo concentrations gradually change, suggesting that regions II and IV are W-rich and Mo-rich solid-solution layers. Six regions (marked I’, II’, III’, IV’, V’, and VI’) are found at the W/Mo interface with the Cu0.25Ni0.75 coating interlayer. Analogously, regions I’, III’, and VI’ are 93W, CuNi layer, and Mo1 base metal, respectively. The graded elemental distributions in regions II’ and V’ are W-rich and Mo-rich solid-solution layers, respectively. The molybdenum and nickel concentrations are uniform between regions III’ and V’ and layered in region IV’. It was reported that a Mo-Ni brittle phase nucleated at the Mo/Ni boundary exists [24]. A MoNi phase was found to be formed at the Mo/Ni/Ta joint interfaces. As a result, the micro-hardness of the joints increased greatly [25]. Therefore, when the Ni content in the Cu1-xNix coating was 75 %, a brittle phase layer was formed at the interface of the W/Mo joints with the CuNi interlayer.

Fig. 5.

Fig. 5.   Microstructure of the W/Mo joints with Cu1-xNix coating interlayers: (a) Cu, (b) Cu0.75Ni0.25, (c) Cu0.5Ni0.5 and (d) Cu0.25Ni0.75.


Fig. 6.

Fig. 6.   Elemental distributions of W/Mo joint with Cu1-xNix coatings interlayer: (a) Cu, (b) Cu0.75Ni0.25, (c) Cu0.5Ni0.5, (d) Cu0.25Ni0.75.


The HRTEM topography results and elemental distributions of the W/Cu0.75Ni0.25/Mo interface are shown in Fig. 7, Fig. 8, Fig. 9. In the dark field TEM image of the joint in Fig. 7(b), the Mo1/Cu0.75Ni0.25 and 93W/Cu0.75Ni0.25 interfaces are clear, without inclusions. The elemental area distributions (as shown in Fig. 7(e-i)) of the interface show that regions I, II, and III are the Mo1 substrate, Cu0.75Ni0.25 interlayer, and 93W substrate, respectively. The Fe atoms in the 93W base metal are inclined to diffuse into the Cu0.75Ni0.25 interlayer. According to Naghavi [26], the diffusion coefficient is D = Do exp(-Q/ kBT), where Do, Q, kB and T are the diffusion pre-factor, the diffusion activation energy, the Boltzmann constant and the temperature, respectively. It has been reported that the value of QW-Cu is 257 ± 21 kJ/mol [27], QW-Ni is 261.4 kJ/mol [28], and QMo-Ni is 281.3 kJ/mol [29]. In contrast, it has been reported that QFe-Cu is 216 kJ/mol [30], and QFe-Ni is 244.15 kJ/mol [31], showing that the values of QW-Cu, QW-Ni, and QMo-Ni are much greater than those of QFe-Cu and QFe-Ni. Consequently, the Fe atoms in the 93W alloy intensively spread into the CuNi interlayers.

Fig. 7.

Fig. 7.   Elemental distributions of W/Mo joint with Cu0.75Ni0.25 coating interlayer: (a) bright field TEM image of interface; (b) dark field TEM image of interface; (c-i) elemental area distributions of diffusion; (h) and (i) elemental distributions across bonded joint marked in (b).


Fig. 8.

Fig. 8.   EDS analysis of W/Mo joint with Cu0.75Ni0.25 coating interlayer marked I, II, III in Fig. 7(a).


Fig. 9.

Fig. 9.   HRTEM images partially of W/Mo joint with Cu0.75Ni0.25 coating interlayer: (a-d) magnified view of area enclosed by white rectangle in Fig. 7(a).


Fig. 8 shows the EDS analysis results for the W/Mo joint with the Cu0.75Ni0.25 coating interlayer marked I, II, and III in Fig. 7(a). Near the Mo1 substrate (region I), the elemental composition is 62.6 % Mo, 20.02 % Cu, 13.68 % Ni, and a little Fe. It can be deduced that the phase in region I may be a Cu and Mo solid solution with Cu, Ni, and Fe atoms dissolved in it. Near the 93W base metal, the elemental composition is 93.37 % W, 7.08 % Ni, 6.03 % Fe, and a small amount of Cu. A W solid solution with dissolved Cu and Ni atoms may be the phase near the 93W. The atomic composition of region II is 69.41 % Cu, 16.55 % Ni, and minute quantities of Fe, Ni, and W, indicating that the phase is supposedly the CuNi solid solution with dissolved Fe, Mo, and W atoms. According to the Mo-Cu and W-Cu binary alloy phase diagrams, the solubility of Cu in Mo and W is very small or negligible. When Fe and Ni are added to the W-Cu and Mo-Cu systems, the mechanical properties of the W-Cu and Mo-Cu composites can be improved [32]. Therefore, copper supplemented with Ni and Fe had improved solid solubility and atomic diffusion with the Mo base metal.

Fig. 9 reveals magnified views of regions 1, 2, 3, and 4 in Fig. 7(a). HRTEM images of the CuNi coating interlayer are shown in Fig. 9(b, c), where misfit dislocations are found to exist in the CuNi coating. Nickel can dissolve in copper and form CuNi solid solution in any proportion. The stacking fault energy of the copper-nickel alloy is larger than that of commercial pure copper; consequently, the copper-nickel alloy possesses excellent mechanical properties [33]. Accordingly, because of the CuNi interlayer, the W/Mo joints could achieve fabulous mechanical properties at low bonding temperatures. Fig. 9(a) reveals the magnified lattice reflection of the cross-section interfacial structure of the Cu0.75Ni0.25/Mo boundary (surrounded by the white blocks (1) in Fig. 7(a)), where three regions can be found. The phase was determined by an analysis of the interplanar spacing. The three regions were a Cu0.75Ni0.25 layer, Cu0.75Ni0.25/Mo diffusion layer, and Mo layer. The high-resolution lattice reflection of the cross-section interfacial structure of the Cu0.75Ni0.25/W boundary (surrounded by the white blocks (4) in Fig. 7 (a)) is displayed in Fig. 9(d), where three regions can be observed. These three regions contain a Cu0.75Ni0.25 layer, Cu0.75Ni0.25/W diffusion layer, and W layer, as proved by an analysis of the interplanar spacing. The lattices of W, Mo, and CuNi are slightly deformed near the boundary of the diffusion layer as a result of the dissolved atoms in these layers. Continuous crystal interfaces were formed at both the Cu0.75Ni0.25/W and Cu0.75Ni0.25/Mo boundaries, showing that exceptional metallurgical bonding was successfully achieved at the interface of the 93W/Mo1 joint with the Cu0.75Ni0.25 coating interlayer.

3.3. Mechanical properties of W/Mo joints with Cu1-xNix coating interlayers

Fig. 10 displays the effect of the Ni content of the Cu1-xNix coating interlayers on the shear strength of the W/Cu1-xNix/Mo joints. With an increase in the Ni content of the Cu1-xNix coating interlayer, the shear strength of the W/Cu1-xNix/Mo joint first increased and then decreased. When the Ni content of the Cu1-xNix coating interlayer was 0%, the shear strength was nearly 200 MPa. In our previous study [14], the Fe-Ni binder in the 93W base metal could diffuse into the Cu film and form a copper solid solution, which promoted atomic diffusion at the Cu film/Mo and W/Cu film interfaces, contributing to the remarkable shear strength of the joints at low bonding temperatures. The average maximum strength of the joint was 316.5 MPa when the Ni content of the Cu1-xNix coating interlayer was 25 %. When Ni was added to the Cu-Mo alloy, the solubility of Mo in Cu increased, and the hardness of the alloys also changed, indicating that the dominant controlling parameter was the heat of the solution [34]. However, as the Ni content of the Cu1-xNix coating interlayer increased, the shear strength of the joint dramatically decreased, as a result of the nucleation and growth of Ni-Mo brittle phases at the interface. It was reported that a Mo-Ni brittle phase nucleated at the Ni/Mo boundary existed [24]. Hence, a small amount of Ni-Mo brittle phase was inferred to nucleate at the Cu1-xNix and Mo interface when the Ni content of the Cu1-xNix was 50 %. From the elemental distributions of the W/Mo joint with the Cu0.25Ni0.75 coating interlayer (shown in Fig. 6(b)), the concentrations of molybdenum and nickel are uniform and layered in region IV’, indicating that a brittle Mo-Ni phase layer is formed at the interface, contributing to a dramatic decrease in the shear strength of the joints.

Fig. 10.

Fig. 10.   Effect of Ni content in Cu1-xNix coating interlayers on shear strength of W/Mo joints.


3.4. Fracture morphology of W/Mo joints with Cu1-xNix coating interlayers

The XRD patterns confirmed the phase constitutions on the fracture surfaces of the tested W/Mo joints with the Cu1-xNix coating interlayers (shown in Fig. 11). When the Ni content of the Cu1-xNix coating was below 50 %, Cu(ss, Ni) (Cu solid solution with dissolved nickel) peaks were observed at the fracture surfaces on both the 93W and Mo1 sides. It can be deduced that Cu(ss, Ni), W, and a very small amount of Mo are the major phases on the 93W sides. The phases on the Mo1 sides are mainly Cu(ss, Ni), Mo, and a little W. Therefore, the W/Mo joints primarily fracture in the Cu1-xNix layer and slightly in the Mo1 and 93W base metals. However, when the Ni content of the Cu1-xNix coating was above 50 %, flimsy MoNi peaks appeared on the 93W fracture surface. Liaw et al. [35] reported that a huge amount of a MoNi intermetallic compound was found in a Mo brazed joint with 70Au-22Ni-8Pd, which decreased the properties of the Mo brazed joint. In the present study, the W/Mo joints mainly formed in the MoNi brittle phase layer and a little in the Cu(ss, Ni) layer and Mo1 base metals, when the Ni content of the CuNi coating was above 50 %.

Fig. 11.

Fig. 11.   XRD patterns of fracture surface of W/Mo joints: (a) 93W sides; (b) Mo1 sides with Cu1-xNix coating interlayers.


Fig. 12 shows the fracture morphology on the 93W side of the W/Mo joints with the Cu0.75Ni0.25 coating interlayer. As displayed in Fig. 12(b), stream flows are observed on the fracture surface, where the main phase is a W solid solution with dissolved Fe, Ni, and Cu atoms (Table 2), suggesting that W grains fracture with a transgranular pattern. Small grains are found (illustrated in Fig. 12(c)), and the main phase in this region is a Mo solid solution with dissolved Ni and Cu atoms (Table 2), indicating that Mo grains disrupt an intergranular model. At the fracture surface of the Mo1 side (Fig. 13(b)), there are large numbers of fine dimples, where the elemental composition is 74.04 % Cu, 24.18 % Ni, and a little Fe (Table 3). The W/Mo joint with the Cu0.75Ni0.25 coating interlayer breaks with the ductile model in the Cu0.75Ni0.25 layer and brittle pattern in the Mo and W base metal.

Fig. 12.

Fig. 12.   Fracture morphology of W/Mo joints with Cu0.75Ni0.25 coating interlayer on 93W side: (a) SEM image; (b, c) magnified SEM micrograph.


Table 2   Elemental composition of the regions marked in Fig. 12.

AreaElemental Composition (at.%)Possible Phases
WMoCuFeNi
162.84-8.025.736.44W(ss, Cu, Ni, Fe)
2-84.4910.83-4.67Mo(ss, Cu, Ni)
3--73.951.5624.49Cu(ss, Ni, Fe)

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Fig. 13.

Fig. 13.   Fracture morphology of W/Mo joints with Cu0.75Ni0.25 coating interlayer on Mo1 side: (a) SEM image; (b, c) magnified SEM micrograph.


Table 3   Elemental composition of the regions marked in Fig. 13.

AreaElemental Composition (at.%)Possible Phases
WMoCuFeNi
1--74.041.7824.18Cu(ss, Ni, Fe)
257.60-19.278.8014.33W(ss, Cu, Ni, Fe)

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Fig. 14 illustrates the fracture morphology and element distribution of W/Mo joints with the Cu0.25Ni0.75 coating interlayer on the 93W side. As shown in Fig. 14(b), a large number of tiny crystal particles appear at the fracture surface on the 93W side, where the EDS analysis shows a composition of 47.04 % Mo, 48.46 % Ni, and little Cu and Fe. Evidently, the atomic ratio of Mo and Ni in this area is nearly 1:1, indicating that the major phase in Fig. 14(b) is the MoNi intermetallic compound. The fracture morphology shows that the fracture pattern is an intergranular fracture. It has been pointed out that the MoNi intermetallic compound is a hard-brittle phase, which weakens the bonding strength of the joints [35]. The fracture pattern of the W/Mo joint with the Cu0.25Ni0.75 coating interlayer is brittle rupture in the MoNi intermetallic compound (IMC) layer.

Fig. 14.

Fig. 14.   Fracture morphology and EDS analysis of W/Mo joints with Cu0.25Ni0.75 coating interlayer on 93W side: (a) SEM image; (b) amplified SEM micrograph; (c) EDS analysis of (b).


Fig. 15 shows schematic diagrams of the microstructure evolution and rupture of the W/Mo joints with different Ni contents for the Cu1-xNix coating interlayer. When the Ni content of the Cu1-xNix coating is below 50 %, the atomic diffusion between the base metals and Cu1-xNix coating interlayer is sufficient, forming solid solutions at the bonding interfaces without any IMCs. The W/Mo joint with the Cu1-xNix coating interlayer breaks with the ductile model in the Cu1-xNix layer and brittle rupture in the Mo and W base metal. When the Ni content of the Cu1-xNix coating is 50 %, a small amount of MoNi IMC particles is supposed to nucleate at the Cu1-xNix coating/Mo1 interface, which decreases the bonding strength of the W/Mo joints. The fractures mostly occur at the CuNi coating layer and Mo1 base metals. When the Ni content of the CuNi coating is 75 %, MoNi particles grow into a thin IMC layer at the Cu1-xNix coating/Mo1 interface. The fracturing occurs at the MoNi IMC layer, and the fracture model is intergranular ductile fracture.

Fig. 15.

Fig. 15.   Schematic diagrams of microstructure evolution and fracture of W/Mo joints with different Ni content in Cu1-xNix coating interlayer: (a) Cu and Cu0.75Ni0.25; (b) Cu0.5Ni0.5 and (c) Cu0.25Ni0.75.


4. Conclusions

In the present study, the effects of the Ni content of the Cu1-xNix coating interlayer on the interfacial microstructure evolution and mechanical properties of W/Mo joints were investigated. The main results are summarized as follows:

(1) When the Ni content of the Cu1-xNix coating was below 50 %, enhanced physical joining and fabulous metallurgical bonding of the W/Mo joints were achieved. As demonstrated by high-resolution observations and an analysis of the joints, the atomic diffusion at the W/Mo joint interfaces adequately formed solid solutions at the bonding interfaces without any IMCs.

(2) With an increase in the Ni content of the Cu1-xNix coating interlayer, the MoNi IMC nucleated and grew at the CuNi coating/Mo1 interface. When the Ni content of the Cu1-xNix coating was 75 %, MoNi particles grew into a thin IMC layer at the CuNi coating/Mo1 interface, leading to micro-cracks at the interface of the W/Mo joints.

(3) With an increase in the Ni content of the Cu1-xNix coating interlayer, the shear strength of the W/Cu1-xNix/Mo joints first increased and then decreased. The maximum average shear strength of the W/Mo joint was 316.5 MPa when the Ni content of the CuNi coating interlayer was 25 %. The Cu1-xNix interlayer promoted diffusion bonding at the W/Mo boundary, which led to remarkable mechanical properties for the W/Mo joint. The formation of a MoNi intermetallic compound at the W/Mo interface weakened the bonding strength dramatically when the Ni content of the CuNi coating was above 50 %.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Nos. 51572208 and 51521001), the 111 Project (No. B13035), and the Joint Fund (No. 6141A02022255).

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